The present invention relates in general to electric heating pads, blankets, pillows, wraps and the like, and in particular to such heating devices which include a plurality of separate low current heating elements.
Present day warming blankets use one continuous wire pair which is threaded in a serpentine pattern throughout the blanket. As a result, this single wire pair must carry the entire electrical current of the blanket. As the current in the wire increases, so does the likelihood of an electrical arc occurring should the wire break or crack. Since the blanket current is sufficient to allow an arc to occur if a wire breaks, the blanket control module must contain safety circuitry to determine when this condition occurs. This is necessary, since an arc could potentially damage the blanket. The control module must detect this fault condition and remove electrical power to eliminate this possibility.
If instead of using one continuous wire pair to construct the blanket, multiple wire pairs were used, then the current in any one wire pair would be low. In this case, if a break occurred in a wire, there would not be enough current to generate an arc. Without the risk of an arc, a break in the wire would not represent a hazard and it would not be necessary to monitor the blanket for this condition. This would simplify the design of the control circuits and thereby reduce cost.
As an alternate design, two wide electrically conductive strips could be used to carry the voltage to multiple PTC heating wires. If the conductive strips were of sufficient size and construction to make breakage virtually impossible, then only the PTC wires would have the possibility of breakage. However, if multiple PTC wires were used, the current in any one PTC wire would be low. Therefore, if a PTC wire broke, there would not be enough current to generate an arc. As with the prior noted design, there would not be a need to monitor for wire breakage.